1. Field of the Invention
The present invention relates to a control apparatus for a flexible manufacturing system (FMS, hereinafter) line.
2. Description of the Prior Art
The physical components of a conventional FMS line are taught in Japanese Patent Publication SHO 60-172449, for example. As shown in FIGS. 11 and 12, conventional FMS lines generally include a plurality of cells U1, U2, . . . , Un disposed in series. Each cell Ui includes a loading station Li, a loading buffer BLi, a machining center Mi, an unloading buffer BVi, and an unloading station Vi. Various kinds of works are successively supplied from a workpiece stocker WS to the FMS line and are finally expelled to a machined workpiece stocker MWS.
In the FMS lines of FIGS. 11 and 12, a workpiece is carried by a robot between the cells and is machined at the machining center Mi. For positioning the workpiece at a machining tool of the machining center, a jig is coupled to the workpiece. The appopriate jig is selected from among various kinds of jigs stocked in a jig stocker S corresponding to the kind of workpiece and the kind of machining center Mi and is carried from the jig stocker S to a loading station Li where the jig is coupled to the workpiece by the robot. The jig with the workpiece is then carried to a machining center Mi of the cell Ui and is machined. The jig with the machined workpiece is then carried to an unloading station Vi of the cell Ui where the jig is separated from the workpiece by the robot. The separated workpiece is then carried to a loading station Li+1 of the next cell Ui+1 by the robot, and the separated jig is returned to the jig stocker S. Carriage of the jig between the cell Ui and the jig stocker S is performed by an automatically guided vehicle (AGV, hereinafter).
Prior art control systems for FMS lines can be generally classified into two types: one shown in FIG. 11 and another shown in FIG. 12.
In the system of FIG. 11, an identification reader I reads not only the presence or absence of a workpiece and/or jig at each station of each cell but also the kind of workpiece and/or jig, when present. Therefore, the ID reader includes a sensor capable of identifying even the pattern of a workpiece and/or jig. In addition, all the workpieces and jigs on the FMS line are tracked, and the positions and kinds of all the workpieces and/or jigs are stored in a process control device C1 constructed of a computer of great capacity. Further, a very complicated electrical communication network is provided between the computer and the ID readers.
In the system of FIG. 12, workpieces are supplied from a workpiece stocker WS to the FMS line in accordance with a predetermined workpiece supply order schedule. Jigs are selected from a jig stocker S and the selected jigs are carried from the jig stocker S to a corresponding station of a cell of the line in accordance with instructions from a production instruction device C2. An AGV performs a predetermined routine along a fixed route.
However, the control systems of FIGS. 11 and 12 include the following problems:
(a) With respect to the system of FIG. 11:
(i) Because the ID reader should include a pattern recognizable sensor, the ID reader is very expensive.
(ii) Because the computer should have a great capacity and the communications network is very complicated, the computer and the network are very expensive.
(iii) Further, when a problem deveplops with the computer and/or the network, much more effort and time is needed for recovery. As a result, the line may be forced to stop for several days before operations can resume. (b) With respect to the system of FIG. 12:
(i) If no jig happens to be present at a station of the cell to which the AGV is moved, the AGV can no longer operate and remain stopped to cause a so-called dead lock.
(ii) For preventing such a dead lock from happening, each station of each cell must be filled with a sufficient number of spare jigs. However, because each jig is machined to very tight dimensions and is very expensive, provision of many spare jigs is not economical.